Method and apparatus for range correction in a radar system

ABSTRACT

A method and apparatus for calibrating range in a radar system. Due mainly to temperature changes in a radar system which cause frequency deviation error, range errors can be introduced into the radar system, thus adversely affecting the determination of the position of targets relative to the host platform. These range errors can be corrected by detecting and accurately estimating the frequency deviation error of a radar system. The present invention improves target position determination performance in a radar system by reducing errors introduced by the frequency deviation error. The present invention relies upon the observation that the Doppler range rate is largely unaffected by frequency deviation error, and thus, is approximately equal to the actual range rate. In accordance with a first range calibration technique of the present invention, the radar system measures the range, Doppler range rate, and azimuth angle of a target during at least two successive time instances. If the measured data is qualified the method corrects the range using a frequency deviation correction factor, K. A second calibration technique of the present invention relies upon both the observation that the Doppler range rate is largely unaffected by frequency deviations and the observation that certain target tracks provide more reliable data than other target tracks. Thus, tracks with more reliable data are given more weight in calibrating the range. The second calibration technique qualifies data, updates a frequency deviation correction factor, and corrects the range using the frequency deviation correction factor. The voltage bias implementation of the present method can be used with either the first or second calibration technique to calibrate the range. In the voltage bias implementation, the range is corrected by adjusting the voltage of the radar system&#39;s RF (radio frequency) source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radar systems, and more particularly tomethods of range correction in radar systems.

2. Description of Related Art

Radio detecting and ranging, commonly referred to as "radar", is usedfor detecting and locating an object of interest, or "target", using thetransmission, reflection, and reception of radio waves. Radar emitsradio waves in a pattern emanating from the surface of the radar'santenna. Typically, radar systems are mounted to a platform such as atower, airplane, ship, automobile or other motorized vehicle. Theobjective of these radar systems is to accurately locate the position ofan object of interest or target relative to the radar's platform.

A number of radar techniques are well known in the art. Radar systemshave been used to determine range, angular position, and range rate ofobjects of interest. Target range and angular position are determined byanalyzing certain properties of the return radio wave signal. Targetrange rate is determined by taking advantage of the well-known Dopplereffect. One distinguishing feature of radar systems is the type ofmodulation technique used to obtain range and range rate data. Examplesof these different radar systems include unmodulated continuous wave(CW) radar, frequency modulated (FM) radar, pulse Doppler radar andfrequency shift keying (FSK) radar. Other distinguishing featuresinclude differences in antenna types and in the approach used inextracting angular information about a target.

Radar locates a target's position by obtaining the target's "azimuthangle" and "range" relative to a reference line or a reference point ofthe radar antenna. A target's azimuth angle is defined as the angulardistance between the antenna reference line and a line extending fromthe radar antenna to the target. A target's range is defined as thedistance from the antenna reference point to the target. Thus defined, atarget's azimuth angle and range yield a calculated target position.Many radar systems analyze frequency domain data from the return signalto calculate the azimuth angle and range of a target's position.However, the calculated range does not always correlate exactly with theactual range. Rather, due to ambient temperature variations, oscillatorvoltage fluctuations, and other well-known causes, errors will occur incalculated range.

Typically, the percent range error, defined as the percent differencebetween calculated range and actual range, is between 10% and 30%.Unless these errors are compensated for by the radar system,inaccuracies can result in the calculation of target positions relativeto the radar system platform. Therefore, it is essential that rangeerrors are accurately estimated and calibrated by the radar system todetermine a target's position precisely.

Radar has been used in a wide variety of platforms to detect theposition of objects. For example, radar has been mounted on "host"automobiles and other host vehicles to detect the position of objects(such as other vehicles) on a road. One such vehicular radar system isdescribed in U.S. Pat. No. 5,302,956, issued on Apr. 12, 1994 to Asburyet al. and assigned to the owner of the present invention, which ishereby incorporated by reference for its teachings of vehicular radarsystems. Another exemplary vehicular radar system using a "monopulse"azimuth radar for automotive vehicle tracking is described in U.S. Pat.No. 5,402,129, issued on Mar. 28, 1995 to Gellner et al. and assigned tothe owner of the present invention, which is also hereby incorporated byreference for its teachings of vehicular radar systems. As describedtherein, object position data has been used in the prior art collisionavoidance systems to brake or steer a host vehicle when the radar systemdetects a potential collision with another vehicle. Alternatively, theradar system may be used in an intelligent cruise control system todecelerate the host vehicle when the radar system detects a potentialcollision with another vehicle and accelerate the host vehicle when thecollision danger terminates.

In both the prior art collision avoidance systems and the prior artintelligent cruise control systems, an accurate calculation of objectposition relative to the radar platform is critical for safe systemperformance. Disadvantageously, due to ambient temperature variationsand oscillator voltage fluctuations, heretofore it has been difficult ifnot impossible to accurately estimate and calibrate the range error.Consequently, the prior art vehicular radar systems disadvantageouslyoften introduced range errors when attempting to determine the positionof targets and therefore introduced undesirable and sometimes dangerousinaccuracies into the collision avoidance process. Therefore, a needexists for a method and apparatus that can accurately estimate the rangeerror and subsequently calibrate the calculated range.

To more fully describe the problems associated with range error,consider the exemplary collision avoidance vehicular radar system shownin FIGS. 1 and 2. As shown in FIGS. 1 and 2, a collision avoidancevehicular radar system 100 is mounted on a host vehicle 12. The hostvehicle 12 is shown in FIGS. 1 and 2 traveling in a direction of travel22 on a road 30. As described in U.S. Pat. No. 5,302,956, the radarsystem 100 cooperates with control systems (not shown) on the hostvehicle 12 in a well-known manner to prevent the collision of the hostvehicle 12 with other objects on the road 30. For example, as shown inFIGS. 1 and 2, the radar system 100 aids the host vehicle 12 in avoidingcollision with other vehicles 40, 50 travelling in front of the hostvehicle 12 in a direction substantially parallel to the direction oftravel 22 of the host vehicle 12. As described below in more detail withreference to FIG. 8, and as disclosed in detail in U.S. Pat. No.5,302,956, the radar system 100 preferably includes a radar antenna 10and a microprocessor or micro-controller 11 (FIG. 8). The radar antenna10 preferably is mounted to a front bumper 13 of the host vehicle 12such that it points in a forward direction substantially parallel to thedirection of travel 22 of the host vehicle 12. The microprocessor 11 inthe radar system 100 calculates the position of objects detected by theradar antenna 10 in a well-known manner as exemplified by the monopulseazimuth radar system described in U.S. Pat. No. 5,402,129.

As shown in FIGS. 1 and 2, the radar antenna 10 includes an antennareference line 20 that is defined by a line emanating from the center ofantenna 10 and perpendicular to the surface of the radar antenna 10. Theradar antenna 10 locates "target" vehicles (e.g., vehicles 40 and 50) ina well-known manner by transmitting a transmission signal (radar beam)having at least two known frequencies, F₁ and F₂. The frequencies areseparated in the frequency spectrum by some pre-defined frequency range.For example, in one typical application, the transmit frequencies areseparated by 300 kHz, although other frequency deviations may be used.The radar system senses the returned transmission signal that isreflected back from the target vehicles. Azimuth angle 19 is calculatedrelative to the antenna reference line 20. For example, in one exemplaryradar system, wherein the radar system 100 comprises a monopulse azimuthradar system (such as that described in U.S. Pat. No. 5,402,129), theradar antenna 10 transmits a transmission signal and senses the returnedtransmission signal that is reflected back from the target vehicles intwo physically separated locations of the radar antenna 10. The radarantenna 10 of a monopulse radar system is split into two antennas (10a,10b) that are physically separated by a few centimeters. This separationof the receive antenna 10 provides a "stereo-vision" perspective to theradar system 100. By comparing selected properties of the reflectedsignals from the two receive antennas, the radar system 100 calculatesazimuth angles to target vehicles in front of the host vehicle 12. Theazimuth angles to the target vehicles are determined relative to theantenna reference line 20.

The radar system 100 determines the closing rate (velocity relative tothe host vehicle 12) of a selected target vehicle in a well-knownmanner. For example, a target's closing rate is determined by analyzingthe well-known "Doppler frequency shift" in the signal returned from thetarget.

The radar antenna 10 includes an antenna reference point 21 that isdefined as a point at the center of antenna 10. Range is calculatedrelative to the antenna reference point 21. The radar system 100determines the range of a selected target vehicle in a well-knownmanner. For example, in one embodiment, the transmission signals F₁ andF₂ are generated using a Frequency Shift Keying (FSK) modulation scheme.The transmission signal F₁ is defined as the carrier frequency and thetransmission signal F₂ is equal to the carrier frequency plus adeviation frequency. In one typical application, F₁ is transmitted at24.7250 GHz frequency whereas F₂ is transmitted at 24.7253 GHz. Thedifference of 300 kHz between F₁ and F₂ is called frequency deviationand it is the stability of this frequency deviation, which influencesthe radar range accuracy. The target range is proportional to thedifference between the phases of returned F₁ and F₂ signals and isinversely proportional to the frequency deviation. Thus, any drift inthe frequency deviation will result in range errors.

As shown in FIG. 2, the radar system 100 preferably determines thelocation of a target vehicle 40 relative to the radar antenna 10 bycalculating both an azimuth angle 19 and a range value of the targetvehicle 40. The azimuth angle 19 is defined as the angular distance fromthe antenna reference line 20 to a target line 24 formed from theantenna reference point 21 to the target vehicle 40. The actual range("R_(a) ") 16 to the target vehicle 40 is defined as the distance fromthe antenna reference point 21 to the target vehicle 40. Ideally, theradar system 100 transmits and receives the signal frequencies withoutany variation in frequency deviation ΔF (i.e., with a completely stablefrequency deviation ΔF). A variation in frequency deviation ΔF isreferred to hereinafter as a "frequency deviance" (i.e., a variation inF₁ -F₂ is referred to as a frequency deviation).

Thus, in an ideal radar system the frequency deviance should equal zero.When the frequency deviance is zero (as shown in FIG. 1), the calculatedrange ("R_(a) ") 17 corresponds exactly to the actual range R_(a) 16.However, due to ambient temperature variations, oscillator voltagefluctuations, and other causes, the frequency deviation ΔF often driftsfrom its nominal value (e.g., in the typical system described above, itdrifts from a nominal value of 300 kHz). Consequently, the frequencydeviance, or the variations in ΔF, often drifts to become a non-zeronumber (i.e., variations in ΔF exist). When the frequency deviation ΔFdrifts from its nominal value (of 300 kHz, for example), a range error15 ("R_(e) ") is introduced (see FIG. 2). The range error R_(e) isdefined as the difference between the calculated range R_(c) 17 and theactual range R_(a) 16 at a given time. For example, if the frequencydeviation's nominal value is 300 kHz and has drifted to 400 kHz due tochanges in ambient temperature or other factors, the range calculationswould have a relative range error of +33%. An exemplary range errorR_(e) 15 caused by frequency deviance (i.e., a drift in frequencydeviation from its nominal value) is shown graphically in FIG. 3 for atarget moving away from the radar.

As shown in FIGS. 2 and 3, because of errors and variations in thefrequency deviation, a range error R_(e) 15 is introduced into thetarget's calculated range R_(c) 17 at each time instant, T, which leadsto target miscalculations. Referring to FIG. 1, in the absence offrequency deviance-induced range errors (i.e., the ideal case wherein nofrequency deviation errors exist and therefore no range error R_(e) 15(FIG. 2) is introduced), the radar system 100 accurately determines theposition of the target vehicle 40 by calculating the actual range R_(c)17 and azimuth angle 19 of the target vehicle. Unfortunately, as shownin FIG. 2, frequency deviance creates the range error R_(e) 15.Consequently, the prior art radar systems disadvantageously miscalculatethe position of the target vehicle as being located in the incorrectposition shown in FIG. 2 as phantom target vehicle 40' (i.e., R_(c) isshortened to the incorrectly calculated range shown in FIG. 2).

Referring to FIG. 2, due to errors in the frequency deviation, the radarsystem 100 miscalculates the range of the target 40 as having a phantomcalculated range R_(c) 17. Thus, the radar system 100 dangerouslyidentifies the target vehicle 40 as being at the position of phantomvehicle 40' having a calculated range R_(c) 17, rather than as being atthe true position of vehicle 40 having an actual range R_(a) 16. Thismiscalculation creates a very dangerous situation for collisionavoidance systems. False alarms are generated when the radar system 100mistakenly determines that a target vehicle is in the host vehicle'sdirection of travel when, in fact, it is not. These false alarms cancause sudden braking and unnecessary steering of the host vehicle 12,which can lead to collisions with the target vehicle or other objects onthe road 30.

False alarms can also create a nuisance condition for the operator ofthe host vehicle 12. The false alarms caused by the range error R_(e) 15can cause the operator of the host vehicle 12 to lose faith in thereliability of the radar system 100 and render the system ineffectivefor warning the operator of real threats. In addition, such false alarmsare distracting and disturbing to the operator.

The range errors caused by variances in ΔF (i.e., the frequencydeviance, variations in the frequency deviation F₁ -F₂) in the radarsystem 100 can be corrected either electrically or mathematically. Thefrequency deviance can be corrected electrically by adjusting thefrequency of the radar system 100. Alternatively, the frequency deviancecan be corrected mathematically by accounting for the taking it intoaccount when calculating the calculated range R_(c) 17. However,regardless of the correction method used in determining the location oftargets, it is essential to detect the presence of frequency deviationvariation, or frequency deviance. Once detected, it is essential toaccurately estimate the frequency deviation variation and to calibratethe radar system accordingly. To date, the prior art systems haveprovided no solution for the range errors that were introduced byfrequency deviance-induced errors.

Accordingly, a need exists for a simple, inexpensive solution to theproblem of detecting, estimating, and calibrating the range errorsintroduced by frequency deviation variations in a radar system. Morespecifically, a need exists for a method and apparatus that can detect,accurately estimate, and compensate for errors introduced by frequencydeviation variations in a radar system. Such a method and apparatusshould be simple to implement, inexpensive, and should work withexisting radar systems. The present invention provides such a solution.

SUMMARY OF THE INVENTION

The present invention is a novel method and apparatus for rangecalibration in a radar system. Due to frequency deviation variationsoccurring in a radar system mainly due to ambient temperature changes,range errors can be introduced when determining target positions. Therange errors can be corrected by detecting and estimating frequencydeviation error in the radar system. The present invention provides amethod and apparatus for detecting and accurately estimating frequencydeviation errors in a radar system and calibrating the range errorsaccordingly. The present invention improves target location performancein a radar system by reducing the errors introduced by frequencydeviation error. The present invention relies upon the observation thatthe integrated Doppler range rate is largely unaffected by frequencydeviation error, and thus, is approximately equal to the actual rangechange. Two fairly simple and easily implemented range calibrationtechniques are described.

In accordance with a first range calibration technique of the presentinvention, the radar system measures the range, Doppler range rate(i.e., actual range change), and azimuth angle of a target during atleast two successive time instances, t=t_(n) and t=t_(n+1). If themeasured data is qualified, the method corrects the range by updating afirst order filter to obtain a frequency deviation correction factor, K,in accordance with the following mathematical formula:

    R.sub.c =K*Range;

where

K=K_(old) +0.25*(1-C), and

K_(old) =previous value for K; and ##EQU1##

The second calibration method of the present range calibration inventionrelies upon both the observation that the Doppler range rate isunaffected by frequency deviation variation (or frequency deviance) andthe observation that certain target tracks provide more reliable datathan other target tracks. Thus, tracks with more reliable data are givenmore weight in calibrating the range. Factors, such as the variance, V,and the number of track data updates, k, determine the reliability of atarget track. Target tracks with either a small variance V or a largenumber of track data set updates k are presumed more reliable and, thus,are given greater weight. The second calibration method qualifies data,updates a first order filter to obtain a frequency deviation correctionfactor, FFDE(n+1l), and corrects the range using the frequency deviationcorrection factor FFDE(n+1) in accordance with a variation of thefollowing mathematical formula depending upon the reliability of thedata:

    FFDE(n+1)=(1-G)*FFDE(n)+G*FDEM(n),

where,

FDEM(n)=the n^(th) frequency deviation error measurement;

FFDE(n)=the n^(th) estimate of the filtered frequency deviation error;and

G=the filter gain (either a constant or a function of V and k).

The voltage bias implementation of the present method can be used witheither the first or second calibration technique. In the voltage biasimplementation, the range is corrected by adjusting the voltage of theradar system's RF (radio frequency) source. The voltage biasimplementation method relies upon the observation that the main cause offrequency deviation in a radar system is ambient temperature change.Also, the present implementation method relies upon the observation thatthe frequency deviation of the radar system can be accurately changed byadjusting the voltage to its RF source.

The voltage bias implementation method uses a look-up table containingRF source frequency deviation correction factors corresponding to avariety of ambient operating temperatures. Initially (i.e., when theradar system is first powered up), the look-up table contains pre-setdefault values containing RF source frequency deviation correctionfactors in a temperature table which span a large temperature range.Then, the voltage bias method samples the ambient temperature atpre-determined time periods. The voltage bias method calibrates therange by adjusting the voltage level of the RF source according to thefrequency deviation correction factor in the look-up table's temperatureentry corresponding to the current ambient temperature.

After the system is allowed to reach a steady state temperature andtargets are available, the voltage bias implementation method updatesthe look-up table's frequency deviation correction factors. The methodupdates the look-up table using the second range calibration technique.When the frequency calibration technique determines a qualifiedfrequency deviation correction factor for the current ambienttemperature, the frequency deviation correction factor value for thecurrent temperature bin is replaced by the qualified frequency deviationcorrection factor. Concurrently, the radar system adjusts the frequencyof the RF source.

The details of the preferred and alternative embodiments of the presentinvention are set forth in the accompanying drawings and the descriptionbelow. Once the details of the invention are known, numerous additionalinnovations and changes will become obvious to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the geometric relationship between a host vehicle and atarget vehicle, wherein the host vehicle has a radar system and a radarantenna mounted thereon. FIG. 1 shows a condition assuming that theantenna frequency deviation variation is zero.

FIG. 2 shows the geometric relationship between the host and targetvehicles of FIG. 1, wherein the antenna frequency deviation variation isnon-zero.

FIG. 3 is a graphic representation of the actual range, calculatedrange, and range errors over time in a vehicular radar system havingfrequency deviation errors.

FIG. 4 is a graphic representation of the Doppler range rate showingrange versus time of a vehicular radar system having frequency deviationerrors.

FIG. 5 shows a flowchart of the general technique of the rangecalibration technique.

FIG. 6 shows a flowchart of a first range calibration technique of thepresent invention.

FIG. 7 shows a flowchart of a second range calibration technique of thepresent invention.

FIG. 8 shows a block diagram of a radar system adapted for use with therange calibration method and apparatus of the present invention.

FIG. 9 shows an exemplary look-up table of frequency deviationcorrection factors used on one preferred embodiment of the presentinvention.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the preferred embodiment and examples shownshould be considered as exemplars, rather than as limitations on thepresent invention.

The range correction method and apparatus of the present inventionincreases the accuracy of locating targets in radar systems. Severalembodiments of the present method and apparatus are described. Forexample, two exemplary methods for estimating frequency deviationvariation which causes a range error are presented below. These twomethods can be independently executed within a radar system to improvethe target detection accuracy of the system. The choice of which methodto use will depend upon the specific system requirements and availableresources within a particular radar system. Alternatively, both methodscan be concurrently used by a radar system to verify the resultsgenerated by each method, thus providing an improved target detectionsystem having location determination redundancy. It will be obvious toone skilled in the radar and target detection art that alternativemethods of determining the range error may be used without departingfrom the scope of the present invention. In addition, although the rangecorrection method and apparatus of the present invention is described asbeing used in a vehicular radar system for collision avoidance, thepresent invention is contemplated for use in any radar platform wheretarget detection and location determination is desirable.

FIGS. 1 and 2 show a graphic representation of the corrections to targetrange that are necessary in a vehicular radar system adapted for usewith the present invention. As shown in FIGS. 1 and 2, a vehicular radarsystem 100 is deployed on a host vehicle 12. As described in more detailbelow with reference to FIG. 8, the vehicular radar system 100 issimilar to prior art vehicular radar systems with the exception that thesystem is adapted for use with the present invention. One exemplaryprior art radar system is described in U.S. Pat. No. 5,402,129, issuedon Mar. 28, 1995 to Gellner et al. U.S. Pat. No. 5,402,129 is assignedto the owner of the present invention and is herein incorporated byreference. Any other convenient radar system can be utilized by thepresent invention provided that the radar system 100 is capable ofdetermining the range, azimuth angle 19, and range rate of detectedtargets. Using the method and apparatus of the present invention, thevehicular radar system 100 aids the operator of the host vehicle 12 indetecting and avoiding collisions with objects that present a danger tothe host vehicle 12. The present invention improves the accuracy andreliability of target detection and location determination functionsperformed by the vehicular radar system 100 and thereby improves theoverall collision avoidance performance of the radar system 100.

Two primary functions are performed by the range correction method andapparatus of the present invention: (1) a frequency deviation error isdetected and accurately estimated; (2) the radar system 100 iscalibrated so that the calculated range, R_(c), is corrected in light ofthe detected frequency deviation error. In one embodiment, the radarsystem may be calibrated by adjusting the frequency of the RF source.This adjustment can be accomplished by adjusting the RF source voltagelevel so that the frequency deviation error becomes small (i.e., thefrequency deviation error is close to zero). Alternatively, the radarsystem may be calibrated mathematically by compensating for thefrequency deviation error when the radar system 100 performs its targetrange and azimuth calculations. Using this latter approach, as describedin more detail below with reference to FIG. 2, the calculated rangeR_(c) is preferably multiplied by a frequency deviation correctionfactor to calibrate the radar system 100. Using the former approach, theRF source frequency is preferably electrically changed to offset thefrequency deviation of the radar antenna 10.

As shown in FIGS. 1 and 2, the present invention is disposed on the hostvehicle 12 that is shown travelling along a road 30 in a direction oftravel 22. A vehicular radar system 100 is preferably deployed withinthe host vehicle 12 and preferably includes a microprocessor 11 and aradar antenna 10. The method of the present invention is preferablyimplemented using software or firmware instructions executed by themicroprocessor 11 or other data processing or sequencing device disposedwithin the radar system 100. Alternatively, the method can comprisesoftware or firmware instructions that are executed by any convenient ordesirable sequencing device such as a state machine, present state-nextstate discrete logic, or field programmable gate array device. Inanother alternative embodiment, the present range calibration method is"hardwired" into the radar system 100 and implemented using eitherdiscrete logic devices, large scale integrated (LSI) devices, very largescale integrated (VLSI) devices, or application specific integratedcircuit (ASIC) devices. Vehicular radar system 100 may be deployed inany convenient location within the host vehicle 12, such as under thefront hood, under the dashboard, within the interior cab, in the trunk,etc.

The radar antenna 10 is preferably mounted to a front bumper 13 of thehost vehicle 12. Alternatively, the radar antenna 10 may be mounted toany other convenient surface on the host vehicle 12, such as the frontgrill, provided that the radar antenna 10 generally faces in a forwarddirection substantially along in the direction of travel 22 of the hostvehicle. In the preferred embodiment of the present invention, the radarantenna 10 comprises a dual lobe monopulse antenna for transmitting aradar transmission signal and receiving the signals reflected back froma target vehicle such as a target vehicle 40 shown in FIGS. 1 and 2.Alternatively, the radar antenna comprises a single patch array antennacapable of both transmitting and receiving radar transmission signals.Due to the phase of the signals transmitted from the two lobes of themonopulse antenna 10, the transmission signal appears to emanate from asingle location within the radar antenna 10. The transmitted signaltravels from the radar antenna 10 to the target vehicle 40 where it isreflected. The target vehicle 40 reflects the transmission signal backto the two lobes of the radar antenna 10. As described in more detail inU.S. Pat. No. 5,402,129, the radar system 100 determines the amplitudedifference between the received signals sensed by the antenna lobes bydetermining the sum thereof and the difference therebetween and thencomputing a ratio of the sum and difference signals. The radar system100 uses information derived from the sum and difference signals in awell-known manner to determine the measured range and azimuth angle 19of the target vehicle 40. However, due to frequency deviation errorpresent in the system, the calculated range R_(c) computed by the radarsystem 100 is often in error and the position of the target vehicle 40is often miscalculated by the system 100.

Therefore, an important object of the present invention is to accuratelydetect the position of the target vehicle 40 by detecting and estimatingthe frequency deviation error of the radar antenna 10 to compensate fora range error R_(e) 15. As described above with reference to FIG. 3, thefrequency deviation of the radar antenna 10 often deviates creating arange error R_(e) 15 as depicted in FIG. 2. The range error R_(e) 15 isthe difference between the calculated range R_(c) 17 and the actualrange R_(a) 16. The radar system 100 detects transmission signalsreflected back from the target vehicle 40 and received by the radarantenna 10 and uses these signals to calculate a calculated range R_(c)17. However, to correctly determine the position of the target vehicle40, the radar system 100 must determine the frequency deviation error ofthe antenna 10.

In one embodiment, Doppler range rate (target velocity) can be used toaccurately determine the frequency deviation error of the radar antenna10. Doppler range rate is largely unaffected by the frequency deviationerror because Doppler range rate is calculated from the relativefrequency shift between the received frequency signal and thetransmitted frequency signal. That is, the Doppler range rate dependsonly on the relative difference in frequency between the received andtransmitted frequencies. Thus, unlike the range measurement, the Dopplerrange rate measurement is unaffected by frequency deviation errors.

Once the frequency deviation error is estimated, the radar system 100can compensate for the range error R_(e) 15 created by the frequencydeviation error. For example, in one embodiment the radar system 100mathematically corrects the calculated range by multiplying the measuredrange by a frequency deviation error correction factor. In analternative embodiment the method and apparatus of the present inventioncalibrates the calculated range by adjusting the frequency of the RFsource. The frequency of the RF source is adjusted by electricallyvarying its voltage level to offset the frequency deviation error.Methods and mechanisms for varying the voltage of the RF source ofantenna 10 are well known and are therefore not described in moredetail.

In one embodiment of the present invention the radar system 100comprises a monopulse radar including Doppler and monopulse technologyused to obtain range rate, range, and azimuth angle 19. Alternatively,the present invention could utilize a switched-beam, frequency-scanned,or mechanically-scanned radar. Monopulse radar operates with its antennaand energy beam fixed in one position. This allows continuous trackingdata on all targets in the antenna detection range without having tointerrupt the data flow to switch beams or mechanically rotate the radarantenna 10. Typically, monopulse radar systems can measure the azimuthangle 19 over a range from approximately 6 degrees to the left ofantenna reference line 20 (FIG. 1) to 6 degrees to the right of antennareference line 20 (for a total radar beam width of approximately 12degrees). The beam width varies with the power delivered to the radarantenna 10. It will be obvious to one skilled in the radar art that thepresent invention can be used with radar systems having antenna beamwidths greater and smaller than 12 degrees.

As described below in more detail the present range calibration methodand apparatus uses several techniques to estimate the frequencydeviation error. Referring now to FIG. 5, the general technique toestimate the frequency deviation error comprises three steps: (1) obtaindata from the radar system 100; (2) determine if the data is qualified;and (3) calculate the frequency deviation error. In a first embodimentof the present invention, the frequency deviation error is calculatedusing the target range and the Doppler range rate data generated by theradar system 100 from only one target track during a given timeinterval. In a second embodiment, the technique calculates the frequencydeviation error using the target range and the Doppler range rate datagenerated by the radar system 100 from multiple target tracks during agiven time interval. Both techniques rely upon the observation thatDoppler range rate is largely unaffected by frequency deviation error.

A First Calibration Technique for Estimating Frequency Deviation andCorrecting Range

In one embodiment of the present invention, the radar system 100measures the range and Doppler range rate of a single target at severalsuccessive time instances, t, over a time interval, T. The time intervalT is defined as the summation of time instances t (i.e., T=t₁ +t₂ + . ..+t_(n), where n=the total number of time instances). A target can beany object within the detection range of the radar system 100 such as astationary road sign or a target vehicle 40.

The first calibration technique of the present range calibration methodrelies upon the observation that the actual range change can be closelyestimated from the Doppler range rate. Thus, the ratio of the measuredrange change to the actual range change can be used to offset thefrequency deviation of the radar system 100. The measured range changeis defined as the change in range measured by the radar system 100 overthe time interval T. In a preferred embodiment, the time instances t are1/16 seconds apart. However, this specific sampling separation is notmeant to limit the present invention. Rather, the method of the presentinvention can accurately calibrate the range with time instances t atdifferent points than 1/16 seconds. The actual range change is definedas the integral of the Doppler range rate measured by the radar system100 over the time interval T. The measured and actual range changes areestimated by a mathematical model. In one preferred embodiment, themathematical model is a set of recursive equations using the qualifiedoutputs of range and Doppler range rate from a tracked target. Becauseof the time it takes for the mathematical model to converge a trackedtarget must be qualified as an acceptable target for purposes ofcalibrating the range.

Thus, to initially qualify as a candidate for range calibrationpurposes, the target must be within a predetermined target detectionrange and it must have a target range rate (velocity) relative to thehost vehicle 12 that falls within a pre-determined range rate (velocity)range. In addition, the magnitude of the signal reflected back to theradar antenna 10 from the qualifying target vehicle is preferablygreater than a pre-determined threshold value and the turn radius of thehost vehicle 12 is preferably greater than a pre-determined thresholdvalue. In one embodiment, the target range is less than 90 meters (300feet) (closing) and is more than 9 meters (30 feet) (receding), thetarget range rate is greater than or equal to 2.7 m/sec (15 ft/sec), thesignal magnitude is greater than or equal to 100 dB, and the turn radiusof the host vehicle 12 is greater than 450 meters (1500 feet).

This calibration technique can operate using only two targetmeasurements obtained at two different time instances (e.g., at t_(n)and t_(n+1), where n=1) after a target is initially qualified. Theapproach obtains several target range and Doppler range ratemeasurements from a single target track at several successive timeinstances. However, to qualify at each successive time instant, theradar system 100 determines the number of data dropouts. A data dropoutis defined as a data set that contains bad data. One of ordinary skillin the art would recognize methods of determining a data dropout andtherefore this is not described in detail herein. Thus, the data dropoutrate should not exceed a pre-determined amount. For example, in thepreferred embodiment, the data dropout rate should not exceed either twosuccessive data dropouts or two separate instances of two successivedata dropouts.

Should the target data at a successive time instant fail to qualify, theradar system is calibrated using the values from the mathematical modelprovided that the data is finally qualified. Thus, the last two datameasurements must qualify in order for the radar system 100 to calibratethe range. To be finally qualified, the target must be within apre-determined target detection range and it must have a target rangerate (velocity) relative to the host vehicle 12 that falls within apre-determined range rate (velocity) range. In addition, the magnitudeof the signal reflected back to the radar antenna 10 from the qualifyingtarget vehicle is preferably greater than a pre-determined thresholdvalue and the turn radius of the host vehicle 12 is preferably greaterthan a pre-determined threshold value. For example, in the preferredembodiment, the target range is not greater than 90 meters (300 feet)(receding) and is less than 9 meters (30 feet) (closing), the targetrange rate has not changed more than 1.52 m/sec (5 ft/sec) from thevalues observed for at t_(n) =1 and t_(n) =2, the signal magnitude isgreater than or equal to 100 dB, and the turn radius of the host vehicle12 is greater than 450 meters (1500 feet).

A mathematical model comprising a set of recursive equations is used tosmooth the range data such as chi-squared or cubic spline smoothing. Inthe preferred embodiment, the set of recursive equations smooth therange data with least-squared smoothing. Least-squared smoothing of datais well-known in the art and one such method of smoothing is describedin "Introduction to Sequential Smoothing and Prediction", by N.Morrison, McGraw Hill, 1969, pp. 339-369 and is herein incorporated byreference for its technique on least-squared smoothing of data. Thefollowing set of equations provides least-squared smoothing of the rangedata:

    ε(n)=R(n)-[z.sub.0 (n-1)+z.sub.1 (n-1)];           (Equation 1)

where,

R(n) is the measured range at time interval t_(n) ; and ##EQU2##

The following equation sums the Doppler range rate data for the targettrack: ##EQU3## where, R_(dot) (i) is the Doppler range rate at timet=t_(i).

The first range calibration technique of the present invention uses thesmoothed data of Equation 2 and the Doppler range rate of Equation 4 tocalibrate the calculated range.

The first calibration technique preferably executes on any radar systemthat locates target positions by obtaining a target range and a Dopplerrange rate for each target vehicle. In one preferred embodiment, thefirst range calibration technique obtains the target range and targetDoppler range rate data for each target track using a monopulse radarsystem similar to that described in U.S. Pat. No. 5,402,129. However,any convenient radar system can be used in cooperation with the presentcalibration technique providing that the radar system is capable ofdetermining both the range and the Doppler range rate of a target.

The steps necessary to implement the first calibration technique of thepresent invention are summarized as follows:

Step 1--Select a target track and measure an initial target range andDoppler range rate for the target track at two successive timeinstances, t_(n) and t_(n+1) where n=1.

Step 2--Determine whether the target is qualified by being within apre-determined detection range, having a range rate (velocity) greaterthan a pre-determined threshold value, having a signal magnitude greaterthan a pre-determined threshold value, and having a turn radius of thehost vehicle 12 greater than a predetermined threshold value. If thedata is qualified the method smoothes the range data using amathematical model.

Step 3--Obtain more data at successive time instances. Determine whetherthe target data is qualified by determining if the number of datadropouts exceeds a pre-determined value. If the data is qualified, thenthe method continues smoothing the range data using the mathematicalmodel in step 2 and the method repeats step 3. Else, the method proceedsto step 4.

Step 4--Determine whether the last two data measurements are qualifiedby being within a pre-determined detection range, having a range rate(velocity) greater than a pre-determined threshold value, having asignal magnitude greater than a pre-determined threshold value, andhaving a turn radius of the host vehicle 12 greater than apre-determined threshold value. If the data is qualified, then themethod proceeds to step 5. Else, the method discards the data sets,initiates n=0, and returns to step 1.

Step 5--Update a first-order filter and use it to calibrate the range ofthe radar system 100, and return to step 1.

FIG. 6 shows a flowchart of the first range calibration technique of thepresent invention. As shown in FIG. 6, the method begins at STEP 600 byfirst obtaining a data set comprising in part the range and Dopplerrange rate data from the target such as a road sign or target vehicle 40(FIG. 2). Two sets of data are measured and assigned an index number, n.Thus, in one preferred embodiment, a first set of data measurementscomprising time (t=t_(n) =t₁), range, Doppler range rate, signalmagnitude, and turn radius is assigned an index of n=1. Similarly, asecond set of data measurements comprising time (t=t_(n) =t₂), range,Doppler range rate, signal magnitude, and turn radius is assigned anindex of n=2. However, the set of data measurements can also comprisemore measurements such as azimuth angle 19.

The method proceeds to step 602 to determine whether the data meetscertain initial qualifications. If so, the data meets the thresholdcriteria and is smoothed. In one preferred embodiment of the presentinvention, the initial qualifications include the target range, Dopplerrange rate, signal magnitude, and turn radius of the host vehicle 12.However, other parameters may be used to qualify the target data for usein the calibration techniques of the present invention. Further, all ofthe qualifying restrictions can be varied without departing from thescope of the present invention. The values given below are exemplaryonly and were obtained through experimentation. Thus, in one preferredembodiment, the target vehicle 40 at both time t=t₁ and t=t₂ ispreferably at a range of less than 90 meters (300 feet) (closing) andmore than 9 meters (30 feet) (receding). However, the present method canaccurately calibrate the range when the target vehicle 40 is closer orfurther from the host vehicle 12. Also, in the preferred embodiment, theDoppler range rate of the target vehicle 40 at both time t=t₁ and t=t₂preferably exceeds a predetermined threshold value such as 2.7 m/sec (15ft/sec). Again, however, the present method can be used when the targetDoppler range rate is less than 2.7 m/sec (15 ft/sec). In addition, inthe preferred embodiment, the magnitude of the signal reflected backfrom the target vehicle 40 and the turn radius of the host vehicle 12 atboth time t=t₁ and t=t₂ preferably exceed a pre-determined thresholdvalue. For example, in one embodiment, the magnitude of the signalreflected back from the target vehicle 40 preferably exceeds 100 dB andthe turn radius of the host vehicle 12 preferably exceeds 450 meters(1500 feet). However, although these are preferable conditions, thepresent method can accurately calibrate the range when the signalmagnitude is less than 100 dB and the turn radius is less than 450meters (1500 feet).

If the above qualifications of STEP 602 are not met for data set n=1 andn=2, then the method proceeds to STEP 604 where the two data sets arediscarded and the system is initiated to n=0. The method then returns toSTEP 600 to obtain two new sets of data. If the above qualifications aremet for data sets n=1 and n=2, then the method proceeds to STEP 606where the range data is smoothed using a mathematical model.

In a preferred embodiment, at STEP 606 the present method smoothes therange data with the set of least-squared recursive equations describedabove (Equations 1-4). The present method can alternatively calibratethe range using several different types of smoothing using mathematicalmodels such as chi-squared or cubic spline smoothing. Once the targetdata is smoothed, the method proceeds to STEP 608.

At STEP 608 the present method obtains another data set from the targettrack of STEP 600 at a new time instance. In a preferred embodiment, thetime instances are 1/16 seconds apart. However, this is not a limitationof the present invention. The method of the present invention canaccurately calibrate the range using time instances different than 1/16of a second. The data set is indexed with the next highest index numberavailable. For example, if the last index number is n=2, then the dataset of STEP 608 is assigned an index number n=3 with t=t₃. The presentinventive method proceeds to STEP 610 to determine whether the number ofdata dropouts exceeds a pre-determined number.

At STEP 610 the number of data dropouts in the current target trackpreferably cannot exceed either two successive data droputs or twooccurences of two successive data dropouts. However, the present methodcan be used with a less-restricitve pre-determined number of datadropouts. If the number of data dropouts does not exceed thepre-determined number of STEP 610, then the method returns to STEP 606to further smooth the range data using the least-squared smoothingequations (Equations 1-4) and the data set from STEP 608. If the numberof data dropouts exceeds the qualifications of STEP 610, then thepresent method proceeds to STEP 612.

At STEP 612 the present method determines whether the last two data setsmeet certain final qualfications. Thus, if the total number of data setsis N, then data set n=N and n=N-1 are the last two data sets. Forexample, if the total number of data sets is 6, then data set n=6 andn=5 are the last two data sets. In one preferred embodiment of thepresent invention the final qualifications include the target range,Doppler range rate, signal magnitude, and turn radius of the hostvehicle 12. However, other parameters may be used to qualify the targetdata for use in the calibration techniques of the present invention.Further, all of the qualifying restrictions can be varied withoutdeparting from the scope of the present invention. Thus, in onepreferred embodiment, the target vehicle 40 at both times t=t_(N) andt=t_(N-1) is preferably at a range of less than 90 meters (300 feet)(receding) and more than 9 meters (30 feet) (closing). However, thepresent method can calibrate the range when the target vehicle 40 iscloser to or further away from the host vehicle 12. The Doppler rangerate of the target vehicle 40 at both time t=t_(N) and t=t_(N-1)preferably has not changed more than 1.52 m/sec (5 ft/sec) from theDoppler range rate at times t=t₁ and t=t₂. Again, however, the presentmethod can be used when the target Doppler range rate changes more than1.52 m/sec (5 ft/sec) from the initial Doppler range rate. In addition,the magnitude of the signal reflected back from the target vehicle 40preferably exceeds 450 meters (1500 feet) at both times t=t₁ and t=t₂.However, the present method can be used when the signal magnitude isless than 100 dB and when the turn radius is less than 450 meters (1500feet).

If the data of STEP 612 fails to meet the final qualifications, then themethod proceeds to STEP 604 where all the data sets are discarded andthe system is initiated to n=0. The method returns to STEP 600 to obtaintwo new sets of data. If the data of STEP 612 meets the finalqualifications, then the method proceeds to a STEP 614 to update thefrequency deviation correction factor using a first order filter and themethod thereby calibrates the system.

At STEP 614, the first order filter for the target is calculated usingthe smoothed data from STEP 606 (Equations 1-4) and Equations 5-9 below.Equation 5 shows the calculation for the measured range change, ΔR_(m),using the last data update of the recursive Equation 2:

    ΔR.sub.m =N*z.sub.1 (N);                             (Equation 5)

Equation 6 shows the calculation for the actual range change, ΔR_(a),using the last data update of the summation Equation 4:

    ΔR.sub.a =I(N)* Δt;                            (Equation 6)

Equation 7 uses the values from Equations 5 and 6 to determine a gain,C: ##EQU4##

Equation 8 is a recursive first order filter using the gain C andyielding a frequency deviation correction factor, K:

    K=K.sub.old +0.25*(1-C);                                   (Equation 8)

The method calibrates the range of the radar system 100 (FIG. 8) using afirst order filter such as Equation 8 yielding the frequency deviationcorrection factor K. Although Equation 8 is preferred, other first orderfilters may be used in alternative embodiments of the present inventionto accurately calibrate the range of the radar system 100. In thepreferred embodiment of the present invention, Equation 9 below is usedto mathematically correct the range of the radar system 100 by scalingit with the frequency deviation correction factor K:

    R.sub.c =K*Range;                                          (Equation 9)

The system can be calibrated in an alternative manner such as by biasingthe RF (radio frequency) source of the radar system 100 to compensatefor the frequency deviation correction factor K. Other calibrationmethods such as biasing voltages are well known to one of ordinary skillin the art. One exemplary voltage bias method is described in detailbelow.

The method is re-initiated by proceeding to STEP 604 where all data setsare discarded and the system is initiated to n=0. After STEP 604, themethod returns to STEP 600 to obtain two new sets of data.

As described above, in one embodiment, it is preferred that the twoinitial data sets be initially qualified and the last two data sets befinally qualified. In alternative embodiments these restrictions on thetarget vehicle need not be rigidly followed in order to take advantageof the present range calibration method. Although the qualifyingrestrictions on the target vehicle described with reference to thepreferred embodiment may, under some circumstances, limit thequalification of a number of target vehicles, it should be noted thatrange calibration will be infrequently required in a temperateenvironment. Therefore, in a temperate environment, only a few targetsmust qualify over a long period of radar system operation. Consequently,the restrictions described above with reference to the preferredembodiment do not adversely affect the utility of the present invention.

When the calibration method is used in a vehicular radar system theradar system's target range is corrected with a frequency deviationcorrection factor calculated from a first order filter before being usedfor collision warning and cruise control functions. Each newlycalculated frequency deviation correction factor replaces a previouslycalculated frequency deviation correction factor. The calibrationtechnique described above with reference to FIG. 6 is opportunistic inthe sense that many targets are monitored continuously to find aqualifying target. The opportunity for calibration may be explored usinga number of targets concurrently and continuously.

The technique described essentially uses a first order filter toestimate the frequency deviation correction factor or error. This filteruses a constant gain value (e.g., 0.25 in Equation 8). A majorenhancement of this approach is provided by a second technique,described below, in which the filter gain is adaptive to statisticalvariations in multiple target tracks.

A Second Calibration Technique for Estimating Frequency Deviation andCorrecting Range

The second technique of the present range calibration method is similarto the first calibration technique in that it relies upon theobservation that the actual range change can be closely estimated fromthe Doppler range rate. Thus, the ratio of the measured range change tothe actual range change can be used to offset the frequency deviation ofthe radar system 100. However, in contrast to the first calibrationtechnique described above, the second calibration technique uses datafrom multiple tracks during a given time period. This data is used tocalibrate the range. Each track represents a different target such as aroad sign or a target vehicle 40, 50 (FIGS. 1 and 2). The secondcalibration technique also relies upon the observation that certaintarget tracks provide more reliable data than other target tracks.

In accordance with this technique, tracks with more reliable data aregiven more weight in calibrating the range. In the preferred embodiment,the variance, V, of a target track provides information on thereliability of the data set of the target track. Variance V is astatistical term defined as the difference between the individual datapoints of a data set and the mean value of the data points. Thus, targettracks with lower variances are given more weight in calibrating therange because they are presumed to be more reliable. These variances arefound to have chi-squared statistical distribution. In the preferredembodiment, the number of track data set updates, k, providesinformation on the reliability of the data set of a target track. In apreferred embodiment, target tracks with higher k values (i.e., moretrack data set updates) are given more weight in calibrating the range.In sum, more weight is given to a target track with either a smallvariance V or a large number of track data set updates k.

However, other qualifications for increasing the reliability of data canbe used with the second calibration technique of the present invention.

After a target track expires, the target track data set must bequalified to update a frequency deviation correction factor, FFDE. Onepossibility for a target track expiring is when it falls outside thedetection range of the radar system 100. The detection range, usuallylimited by power and other factors, is variable and should not be viewedas limiting the present invention. The second calibration technique ofthe present method updates the frequency deviation correction factorFFDE with the gain G of the expired target track. In one preferredembodiment, the method of determining the gain G depends on the numberof track data set updates k of an expired track. For example, if thenumber of track data updates k is greater than a predetermined numbersuch as 6, then the gain G is a function of both the variance V and thenumber of track data set updates k. This is the case because the trackdata set is sufficiently reliable to calculate the gain G. Else, thegain G is a constant value. Estimates of the variance are unreliable forvery small sample sets. The sample sets are very small when k is lessthan 6. However, the gain G can alternatively be a function of othercharacteristics of the track data set. The gain G is used to update thefirst order filter. The frequency deviation correction factor iscalculated using a first order filter. The range is preferablycalibrated using either a mathematical correction or an RF sourcecorrection.

The steps necessary to implement the second calibration technique of thepresent invention are summarized below as follows:

Step 1--Obtain a data update on a target track and store the data updatein a track data set corresponding to the target track.

Step 2--Determine whether the track has expired (i.e.--no more data isavailable on the track. This may be caused by the target being out ofthe detection range of the radar system 100). If the track has notexpired then the method returns to Step 1 to obtain a data update on anytarget track. Else, the method proceeds to Step 3.

Step 3--Determine whether the number of track data updates k is greaterthan a pre-determined number. If so, then the method proceeds to Step 5.Else, the method proceeds to Step 4.

Step 4.--Update a first order filter with constant gain G using thetrack data set of the expired track. Use the frequency deviationcorrection factor to calibrate the range. The method returns to Step 1.

Step 5--Update a first order filter using a gain G calculated from avariance weighted value of the track data set of the currently expiredtrack. Use the frequency deviation correction factor to calibrate therange. Then, return to Step 1.

FIG. 7 shows a flowchart of the second range calibration technique ofthe present invention. As shown in FIG. 7, the method begins at a STEP700 by first obtaining a track data update for a target track from theradar system 100. In a preferred embodiment, a track data updatecomprises the following:

1) n=track index number for the track data set;

2) k=count of track data updates in track n;

3) R(k,n)=measured range of the n^(th) target track at time t=t_(k) ;

4) dt=time span between the measurement samples;

5) V(k,n)=Doppler range rate of the n^(th) target track at time t=t_(k); and

6) FDEM(k,n)=Frequency Deviation Error Measurement of the n^(th) targettrack at time t=t_(k) ;

In an alternative embodiment of the second calibration technique of thepresent invention the method calculates FDEM(k,n) using the followingformula: ##EQU5## R(k) is the k^(th) estimate for the range to thetarget; V(k) is the k^(th) estimate for the target velocity from theDoppler range rate;

and

dt is the time span between measurement samples.

In a preferred embodiment of the present invention, the time span dt isa constant number. For example, a radar system that measures a targettrack every 1/16 of a second having a track data set update every othertarget track measurement has a time span dt of 2/16 of a second.

In yet another alternative embodiment of the present calibration method,the track data set is updated every 5 target track measurements. Thus,R(k,n) is an average of these five range values and the measured rangerate is calculated by dividing R(k,n) by the time span dt. However, anynumber of updates per track data set update can be used withoutdeparting from the scope of the present invention. For example, a radarsystem having a target track measurement every 1/16 of a second has fivevalues for range over 5/16 of a second. Referring again to FIG. 4, thefive target track range measurement values corresponding to T₀ -T₅ areaveraged to estimate ΔR₁. Similarly, the time spans from T₀ -T₅ areadded together to obtain ΔT₁ (dt). These values are used to calculateR(k,n) as described above.

A track data set for the n^(th) target track consists of k sets of trackdata updates. Thus, a new target track is assigned the next highestunused number as its track index number. When a second track data updateoccurs for an existing target track, the track index number n remainsthe same and k increases to 2. The following example of a preferredembodiment is provided to clarify STEP 700 and the overall operation ofthe second calibration technique:

Initially, the radar system 100 is turned on and contains no targettrack data. After some period of time, the radar system 100 obtains itsfirst track data update. The track data update corresponds to a targetvehicle (e.g., target vehicle 40 of FIGS. 1 and 2). A track data set iscreated and stored containing the first track data update values for n,k, R(k,n), dt, V(k,n), and FDEM(k,n). The target track is assigned a newtrack index number of n=1 because it is the first target track afterinitiating the radar system. The count of updates k for the track dataset corresponding to the target vehicle 40 is assigned the value of k=1.The count of updates k is assigned the value of k=1 because there isonly one track data update for this target track.

The radar system 100 then obtains its second track data update. However,the second track data update corresponds to a second (e.g., the targetvehicle 50 of FIGS. 1 and 2). A track data set is created and storedcontaining the current track data update values for n, k, R(k,n), dt,V(k,n), and FDEM(k,n). The target track is assigned a new track indexnumber of n=2 because it is the second target track after initiating theradar system. The count of updates k for the track data setcorresponding to the second target vehicle is assigned the value of k=1because there is only one track data update for this target track n=2.

The radar system 100 then obtains its third track data update. Similarto the first track data update, the current track data updatecorresponds to the first target vehicle (e.g., the target vehicle 40).The current track data update is added to the track data setcorresponding to the first target vehicle (i.e., n=1) and storedcontaining the current track data update values for n, k, R(k,n), dt,V(k,n), and FDEM(k,n). The target track index number remains n=1 becauseit corresponds to the first target track. The count of updates k for thecurrent track data set is assigned the new value of k=2 because this isthe second track data update for target track n=1 .

At STEP 702, the second calibration method determines whether the targettrack has expired. In one embodiment, a track is determined to beexpired if the track fails to continuously update every 1/16 seconds inorder to allow the value of k to increment for the frequency correctionalgorithm. A target track also expires when it is out of the detectionrange of the radar system 100. In a preferred embodiment, the radarsystem 100 determines when a target track has expired by determiningwhether the target track has updated within a pre-determined amount oftime. However, alternatively the second calibration method of thepresent invention can use other time periods or different methods ofdetermining when a target track is out of the detection range. If thetarget track has not expired, then the present method returns to theSTEP 700 to obtain another track data update. Else, the method proceedsto the STEP 704 to determine whether the number of track data updates kof the track data set of the expired target track exceeds apredetermined number.

At the STEP 704 the present inventive calibration method determineswhether the track data set is sufficiently reliable to calculate thegain G from the target track data set. A track data set with a highernumber of track data updates k is presumed to provide more reliabledata. Thus, the number of track data updates k preferably exceeds apre-determined number. In a preferred embodiment of the secondcalibration method the pre-determined number of track data updates k is6. This value of k is exemplary and has been obtained throughexperimentation. If the number of target track data updates k of theexpired target exceeds 6 the method proceeds to STEP 706 because thetrack has enough data to reliably calculate a gain G from its track dataset. Else, the method proceeds to STEP 708 where the method uses aconstant gain G.

At STEP 706, the method shown in FIG. 7 calculates a frequency deviationcorrection factor FFDE(n+1) using a first order filter. The methodcalibrates the range. The second calibration method performs a series ofcalculations using the track data set values for the expired targettrack. The method calculates the first order filter using the gain G(expressed as G(k,V)), the variance V, and the number of track dataupdates k. In a preferred embodiment, the method of the presentinvention uses the following equations to calculate the frequencydeviation correction factor:

ADEM(n)=the average value of all of the FDEM's for target track n. Thus,

    ADEM(n)=(FDEM(1,n)+FDEM(2,n)+. . .+FDEM(k-1,n)+FDEM(k,n))/k;(Equation 11)

where,

k=the number of target track updates; and

n=the track number;

The sum of 1 through k (FDEM)² is: ##EQU6##

V=the variance of the FDEMs=(Sum of FDEMs Squared-k*(ADEM ))/(k-3).Thus, ##EQU7##

To ensure stability, the gain G is preferably given a value of ##EQU8##A substitiution for x can be made such as x=GS*Vs*k/V, where GS=the gainfor stray targets=1/512 and Vs=the variance for stray data=1/16. Thevalues given for GS and Vs are exemplary only and were obtained throughexperimentation. These values can be varied without departing from thescope of the present invention. Thus, the gain G(k,V) is determined byEquation 14 below: ##EQU9##

Equation 14 is valid if k is greater than 6. Otherwise, G(k,V)=k/512.

The method then calculates a value for a frequency deviation correctionfactor FFDE(n+1) using the gain G(k,V):

    FFDE(n+1)=(1-G(k,V))*FFDE(n)+G(k,V)*ADEM(n);               (Equation 15)

The calibration method of FIG. 7 calibrates the range value at STEP 706.The range is preferably mathematically calibrated. However, the rangecan also be calibrated by adjusting the frequency of the RF source ofthe radar system 100. The range is mathematically calibrated using thefrequency deviation correction factor FFDE(n+1) in accordance withEquation 16 below:

    R.sub.calibrated =Range/FFDE(n+1);                         (Equation 16)

The method returns to STEP 700 to obtain another track data update.

As shown in FIG. 7, when the method determines at STEP 704 that there isinsufficient data reliability in the track data set, the methodcalculates the frequency deviation correction factor FFDE(n+1) at STEP708 using a constant gain G. The radar system is calibrated eithermathematically or using a frequency adjustment on the RF source. Thesecond calibration method performs a series of calculations at STEP 708using the track data set values for the expired target track. The methodcalculates the first order filter using a constant gain G, expressed asGs (the gain for stray targets), and the number of track data updates k.In a preferred embodiment, the method of the present invention uses thefollowing equations to calculate the first order filter:

ADEM(n)=the average value of all the FDEM's for target track n. Thus,

    ADEM(n)=(FDEM(1,n)+FDEM(2,n)+. . .+FDEM(k-1,n)+FDEM(k,n))/k;(Equation 17)

where,

k=the number of target track updates;

n=the track number; and

The calibration method calibrates the range value of the radar system100 at STEP 708. The range is preferably mathematically calculated.However, the range can also be calibrated by adjusting the frequency ofthe RF source of the radar system 100. In the preferred embodiment, therange is mathematically calibrated using the first order filter(frequency deviation correction factor), FFDE(n+1), in accord withEquation 16. As shown in FIG. 7, the method returns to STEP 700 toobtain another track data update.

The radar system's target range is preferably corrected with a frequencydeviation correction factor FFDE(n+1) before it is used for collisionwarning and cruise control functions. Each newly calculated frequencydeviation correction factor replaces a previously calculated frequencydeviation correction factor. The technique described above withreference to FIG. 7 is opportunistic in the sense that many targets maybe continuously monitored until a target track expires. The data fromthe expired track's data set is preferably used to calibrate the range.

FIG. 8 shows a block diagram of a radar system 100 that is adapted foruse with the range calibration method and apparatus of the presentinvention. An understanding of the function and operation of many of thecomponents of the radar system 100 are not essential to understandingthe present invention and therefore are not described in more detailherein. A detailed description of a radar system similar to that shownin FIG. 7 is provided in U.S. Pat. No. 5,402,129. In one embodiment, theradar system 100 is a monopulse radar system and a multiple-frequencymodulated system. The radar system uses a very narrow bandwidth and isthus extremely spectrum efficient. The narrow-band system was developedto operate in the existing FCC Part 15 unlicensed band at 24.725 GHzwith an authorized bandwidth of only 100 MHz. Alternatively, 46.8 GHzand 76.5 GHz can be used to allow bandwidths of 200 MHz and 1000 MHz,respectively. The frequency and bandwidth are variable and are notlimitations of the present invention. In one embodiment, the antenna hasa 3 dB half-power beamwidth that is 6 degrees in azimuth and 5 degreesin elevation with an antenna aperture size of 15 cm high by 20 cm wide.Other configurations can be used to practice the present invention.

As described above, the radar system 100 preferably includes a radarantenna 10 and a microprocessor or micro-controller 11. The rangecalibration techniques described above with reference to FIGS. 1-7preferably comprise software instructions that are executed by themicroprocessor 11 in the radar system 100. Alternatively, thecalibration techniques may be implemented in firmware or hardwarespecifically designed for the purpose. Any convenient means forimplementing the calibration techniques described above may be used bythe radar system 100 without departing from the scope of the presentinvention.

Voltage Bias Implementation of the Present Method

Both the first and second calibration techniques described above withreference to FIGS. 6 and 7 can calibrate the radar system by adjustingthe frequency of the radar system's RF (radio frequency) source (e.g.,Gunn Diode). The voltage bias implementation method relies upon theobservation that the frequency of the radar system 100 can be accuratelychanged by adjusting RF source voltage values. The present method alsorelies upon the observation that the main cause of frequency deviationin a radar system 100 is ambient temperature change.

The voltage bias implementation preferably uses a look-up tablecontaining RF source frequency deviation correction factorscorresponding to a variety of ambient operating temperatures. Initially(i.e., when the radar system is first powered up), the look-up tablecontains pre-set default values containing RF source frequency deviationcorrection factors (obtained through experimentation) in temperaturebins which in total span a large temperature range (such as between 40and 95 degrees Celsius). The voltage bias method samples the ambienttemperature at pre-determined time intervals (e.g., once per minute).The present method calibrates the range by adjusting the frequency ofthe radar system 100 when the current ambient temperature changes to adifferent temperature bin. The voltage bias method calibrates the rangeby adjusting the frequency of the radar system 100. One method ofadjusting the frequency of a radar system is to adjust the voltage levelof the RF source. This voltage adjustment should be made in accordancewith the frequency deviation correction factor in the look-up table'stemperature bin corresponding to the current ambient temperature. Othermethods of adjusting the voltage of the RF source and thus changing theradar's frequency will be obvious to one of ordinary skill in the art.

After the system is allowed to reach a temperature steady state bywaiting a pre-determined waiting period, the present method updates thelook-up table's pre-set frequency deviation correction factors. Themethod can update the look-up table through any frequency deviationcalibration technique such as the first or second calibration techniquesdescribed above. When the frequency calibration technique determines aqualified frequency deviation correction factor for the current ambienttemperature, the frequency deviation correction factor value for thecurrent temperature bin is replaced by the qualified frequency deviationcorrection factor. Concurrently, the radar system adjusts the frequencyof the RF source using the ambient temperature sampling method asdescribed above.

The present method preferably includes a microprocessor 11 having accessto a memory. The method of the present invention preferably comprisessoftware or firmware instructions executed by the microprocessor 11(FIG. 8) or other data processing or sequencing device disposed withinthe radar system 100. In another alternative embodiment, the presentrange calibration method is "hardwired" into the radar system 100 andimplemented using either discrete logic devices, large scale integrated(LSI) devices, very large scale integrated (VLSI) devices, orapplication specific integrated circuit (ASIC) devices.

In a preferred embodiment, the RF source is a Gunn diode and the look-uptable contains twenty-eight bins, each bin ranging 5 degrees Celsiusfrom -40 degrees to 95 degrees Celsius. However, a different number ofbins and bin ranges can be used without departing from the scope of thepresent method. In one embodiment, at system initialization, the presentvoltage bias method initializes the look-up table with pre-set defaultvalues of frequency deviation correction factors corresponding to eachof the twenty-eight temperature bins. These frequency deviationcorrection factors are variable and may be obtained throughexperimentation. The look-up table is preferably stored in memory. Anexemplary look-up table is shown in FIG. 9. The actual entries to thelook-up table shown in FIG. 9 are exemplary only, as they may change inactual use of the system. Alternatively, a customized look-up tableusing values that are manually entered may be used. In yet anotheralternative embodiment, a database that represents different Gunn diodescan be used.

After system initialization, the voltage bias method preferably samplesthe ambient temperature at a rate of once per minute. The rate of onceper minute is exemplary and the present method can be accomplished usinga different sampling rate. The present method calibrates the range byadjusting the frequency of the radar system 100 when the current ambienttemperature changes to a different temperature bin. The voltage biasmethod calibrates the range by adjusting the voltage level of the GunnDiode according to the frequency deviation correction factor in thelook-up table's temperature bin that corresponds to the current ambienttemperature. In a preferred embodiment, the present method adjusts thevoltage level of the Gunn diode by adjusting the setting of adigital-to-analog converter (DAC) that controls the voltage level of theGunn diode. However, other methods of adjusting the voltage level of anRF source of a radar system 100 will be obvious to one of ordinary skillin the art.

In a preferred embodiment, the pre-determined waiting period to allowthe radar system 100 to reach a steady state temperature is 5 minutes.After this 5 minute waiting period the voltage bias implementationmethod of the present invention preferably adjusts the frequency of theGunn diode when either the ambient temperature changes to a newtemperature bin (as described above) or the current frequency deviationcorrection factor is updated by a calibration technique.

The voltage bias implementation method of the present inventionpreferably uses a microprocessor having access to memory or similardigital storage logic devices. Initially, the voltage bias method uses alook-up table with pre-set frequency deviation correction factor valuescorresponding to temperature bins. The voltage bias method updates thelook-up table through a frequency calibration technique such as thesecond calibration technique described above. Whenever the voltage biasmethod senses an ambient temperature switching to a differenttemperature bin or a frequency deviation correction factor update, thesystem implements a hardware correction of the range by adjusting thevoltage of the RF source. In an alternative embodiment, a mathematicalcorrection implementation method can be used by simply using the look-uptable's frequency deviation correction factors in the same manner as thevoltage bias method, but correcting the range mathematically instead ofvarying the RF source's voltage.

In summary, the present invention includes a method and apparatus forcalibrating range in a radar system. The present invention isparticularly useful in automobiles, trucks, vans, or any other vehiclethat travels on a road with other vehicles. The present inventionreduces system size and costs by not requiring the use of additionalequipment for range calibration purposes. The present inventionadvantageously uses relatively simple and straight-forward algorithmsfor calculating a frequency deviation correction factor. The calibrationtechniques described above may be used independently, or concurrently,to accurately calibrate the range of a radar system.

A number of embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the present invention may use any FCC-approved vehicle radarfrequency and bandwidth. As described above, the present inventionpreferably uses a monopulse radar system using at least two transmittingfrequencies. However, the present invention is not so limited. Theinvention contemplates use with any radar system that can determinerange, range rate, and azimuth angle 19 of targets. For example,switched beam, frequency scanned, or mechanically scanned radar systemsmay be employed. In addition, the present invention may alternatively beused in a radar system.

Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiment, but only by the scope ofthe appended claims.

What is claimed is:
 1. An apparatus for calibrating range in a radarsystem, comprising:a) a radar antenna wherein the antenna outputs aplurality of range values and a plurality of Doppler range rate values;and b) a microprocessor, operatively connected to the antenna, wherein afrequency deviation correction factor is generated by the microprocessorand the range of the radar system is calibrated according to thefrequency deviation correction factor.
 2. An apparatus for calibrating arange measurement in a radar system, comprising:a) selecting means forselecting a calibration target for use in calibrating a rangemeasurement; b) initiating means, responsive to the selecting means, forobtaining an initial measurement of a target range and a Doppler rangerate at a first time instance; c) measuring means, responsive to theselecting means, for obtaining subsequent measurements of the targetrange and the Doppler range rate at successive time instances subsequentto the first time instance; d) qualifying means, responsive to themeasuring means, for qualifying the calibration target selected by theselecting means; e) calculating means, responsive to the qualifyingmeans, for calculating a frequency deviation correction factor using theinitial and subsequent target range and Doppler range rate measurements;and f) calibrating means, responsive to the calculating means, forcalibrating the range to compensate for frequency deviation in the radarsystem.
 3. A method of calibrating a range measurement in a radar systemcomprising an antenna and a computer, comprising the steps of:a)selecting a calibration target for use in calibrating the rangemeasurement; b) obtaining an initial measurement of a target range and aDoppler range rate at a first time instance; c) obtaining subsequentmeasurements of the target range and the Doppler range rate atsuccessive time instances subsequent to the first time instance; d)qualifying the calibration target; e) calculating a frequency deviationcorrection factor caused by a frequency deviation using the initial andsubsequent target range and Doppler range rate measurements obtained atSTEPS b) and c); and f) calibrating the range to compensate for thefrequency deviation.